Many modern-day optical systems, including optical communication systems, utilize semiconductor lasers in combination with optical fibers. Optical coupling between the laser and the fiber is usually accomplished by an intervening optical system (e.g., a lens) or by placing an end of the fiber immediately adjacent the output end of the semiconductor laser.
Unfortunately, the coupling efficiency between a typical semiconductor laser and an optical fiber tends to be poor because light emitted from a semiconductor laser has an elliptical or astigmatic cross-section due to the asymmetric waveguide structure of the laser. An optical fiber, on the other hand, has a symmetric waveguide structure and so has a symmetrical acceptance (solid) angle. Further, for single-mode fiber, the acceptance angle is small, typically only a few degrees.
Stated in different terms, the waveguide modes of the optical fiber are symmetric while the waveguide modes of a typical diode laser are asymmetric, and this mode mismatch results in coupling loss. The mode mismatch is even greater when the laser lases in multiple lateral modes and the optical fiber supports only a single lateral mode. It is therefore desirable to have the semiconductor laser lase in a manner that allows for efficient coupling into an optical fiber.
Semiconductor ridge lasers, which are known for their high output power, are often employed in applications involving single mode optical fiber. Efficient light coupling from a semiconductor ridge laser into a single mode optical fiber is facilitated by making the output beam profile from the ridge laser as symmetric as possible. Further, the laser preferably lases in a single lateral mode that matches, to the extent possible, the lateral mode of the fiber (which for most single mode fibers is described by a Bessel function).
To obtain a high output power from the semiconductor laser, a high injection current is required. However, there is a beam-steering effect associated with high injection current that causes the output beam to diverge. This effect is commonly called “kink” because it manifests itself as a kink in the optical power v. injection current characteristic curve (often referred to as an “L-I” curve) of the semiconductor laser. The kink in the L-I curve and the associated output light divergence is due to the interaction between the first and fundamental lateral modes of the semiconductor laser, and the fact that the lateral mode structure in the laser is a function of the refractive index of the waveguiding structure.
At low injection current, the laser typically operates in the fundamental (i.e., zeroeth order) lateral mode, and the refractive index of the ridge waveguide remains constant. However, at a high injection current, the mode structure becomes unstable due to ohmic heating and spatial hole burning. These changes allow the first order lateral mode to propagate and resonate with the fundamental lateral mode, which causes kink. As the first order lateral mode has a higher divergence angle than the fundamental mode, it does not couple into a single-mode optical fiber as efficiently.
The prior art includes techniques for altering the shape of the output beam of a semiconductor ridge laser by providing a taper at the output end of the ridge. For example, U.S. Pat. Nos. 6,052,397 and 6,174,748 B1 disclose a laterally and vertically tapered ridge structure that transforms a highly elliptical mode profile in an active gain section of a semiconductor ridge laser into a substantially circular mode profile in a passive waveguide section of the device. However, a shortcoming of this type of device is that the passive waveguide section supports multiple higher-order lateral modes that in the output beam diverge significantly with respect to the fundamental mode. Thus, the coupling efficiency between the semiconductor laser and the fiber is not optimized. Also, the portion of the laser associated with the tapered ridge section is passive, so that it does not contribute to increasing the output power of the laser.
Other related prior art devices provide for amplification of the output light in a tapered end of the ridge. An example of such a device is a master oscillator power amplifier (MOPA). In a MOPA device, the tapered region is an active gain medium, so that the light is amplified over a larger area of the taper. Further, the tapered output facet is antireflection coated so that only amplification and not lasing occurs in the tapered section. While a MOPA device provides a higher output power by virtue of the amplification in the tapered end, the tapered end also supports multiple higher-order lateral modes, which as mentioned above, can create a higher output divergence resulting in reduced coupling efficiency.